Appropriate solid state gain media for such a device must have absorption lines in the blue-violet wavelength region. Different crystalline host materials doped with rare earth ions have been already proposed as gain media for such solid state laser devices. The Pr3+-ion is of great interest in this context since it shows absorption at the typical emission wavelength of GaN-laser diodes and can convert this radiation into laser emission at blue, green, red and orange wavelengths.
U.S. Pat. No. 6,490,309 B1 discloses a solid state laser device in which a Pr3+-doped crystal is optically pumped by a GaN-based laser diode. The document proposes different Pr3+-doped crystals to obtain laser emission from Pr3+ at several visible wavelengths. The exemplary embodiments are based on Pr3+-doped LiYF4 (YLF) crystals. Only one of the host materials listed in this document, the Y3Al5O12 crystal (YAG), is a cubic crystal.
However, Cheung et al. in “Excited-state absorption in Pr3+:Y3Al5O12”, Physical Review B, Vol. 49, No. 21, pages 14827-14835 demonstrated that exited state absorption from the 3P0 energy level rules out the possibility of using Pr3+:YAG as a solid state laser crystal in the visible and near ultraviolet spectral region.
It is well known that rare earth ions placed in crystalline host materials show narrow spectral absorption lines. FIG. 1 shows an example of the absorption line of Pr3+:LiYF4 at room temperature and for two different polarizations with respect to the crystal axis. On the other hand, the emission of GaN-based laser diodes exhibits a wavelength shift related to the temperature of the laser diode and to the diode current. Moreover, the fabrication process leads to different emission wavelengths with a peak centered between 440 and 450 nm. Measurements of the spectral shift of the laser emission of a commercially available GaN-based laser diode for different diode currents and a fixed laser diode base plate temperature of 25° C. show that the wavelength for the maximum emission varies by about 2 nm for the diode current varying from 200 mA to 500 mA. This corresponds to an output power variation from 110 mW to 480 mW. The spectral shift for a fixed diode current and a laser diode base plate temperature variation between 20 and 30° C. is about 0.5 nm for 10° C. The emission wavelength of this GaN-based laser diode varies between 443.7 nm at 20° C. and for a diode current of 110 mA (just above the threshold of the laser diode) and 446.5 nm at 30° C. and for a diode current of 500 mA (maximum specified current for this laser diode). Due to the narrow spectral absorption lines of the gain media and the temperature and current dependence of the maximum emission of GaN-based laser diodes the corresponding solid state laser devices show a strong temperature sensitivity. This makes an efficient and stable optical excitation of Pr3+-doped host materials by means of GaN-based laser diodes difficult.